In the fabrication process for semiconductor devices, numerous fabrication steps, as many as several hundred, must be carried out on a silicon wafer in order to complete the circuits needed for the devices. Since the processing of silicon wafers requires extreme cleanliness in the processing environment and that no contaminating particles or films are allowed, the surface of the wafer must be cleaned after each processing step. For instance, it is cleaned after the deposition of a coating layer such as oxide or after the formation of a circuit in a processing step such as etching. A frequently used method for cleaning the wafer surface is wet scrubbing.
In a wet scrubbing method, a wafer is rotated at a high speed, i.e., at least 200 RPM and preferably 1,000 RPM with a jet of high pressure deionized water sprayed on top. The water jet is normally sprayed at a pressure of about 2,000.about.3,000 psi. The water movement on the top surface of the wafer displaces any contaminating particles that are lodged on the wafer surface. One limitation of the water jet scrubbing method is that the process only moves particles from side to side in openings, such as oxide windows, without actually removing the particle. Furthermore, as image size decreases, it becomes more difficult for water to reach the particles in openings because of increased surface tension.
It has also been noted that in a water jet scrubbing process conducted on a silicon wafer that is coated with an insulating material, i.e., an oxide layer as an inter-metal dielectric layer, some regions of the film is damaged at the wafer center by the cumulated stress from the water jet when the aperture size of the jet nozzle is too large or is distorted. The damaged film can be identified by a KLA scan, even though a large number of wafers must be tested since the probability of such damage is only about 10.about.30%. This is shown in FIGS. 1 and 2.
FIG. 1 shows an illustration of a silicon wafer surface that is scanned in a conventional water jet scrubbing method. Wafer 10 is normally positioned on a wafer platform (not shown) situated in a scrubbing apparatus and rotated at a predetermined rotational speed. A suitable rotational speed may be between 200 RPM and 2,000 RPM. The centrifugal force acting on the water flow on the wafer surface removes contaminating particles or films. The jet of deionized water which has a water pressure of approximately 50 kg/cm.sup.2, is scanned on top of the wafer surface along trace 12 which normally runs through wafer center 14. The wafer surface is scanned by the water jet at least once, and preferably several times.
A KLA scan on a wafer surface coated with an oxide film layer and scanned by a high pressure water jet is shown in FIG. 2. The black dots shown on the surface of the wafer indicate stress defects that have formed under the water jet pressure.
It has been noted that the stress defects only occur on certain types of surface coating layers and only for certain thicknesses of layers coated on a wafer surface. In the conventional water jet cleaning method, as shown in FIG. 1, it is difficult to identify which type of films will be damaged since the defects or damages are occurring only randomly at the wafer center. Furthermore, it is difficult to monitor whether the aperture in the jet nozzle is distorted or deformed.
A further complication in determining a cause for a surface defect in a coated film is that the defect may be caused by electrostatic discharge (ESD) damage which may have the same appearance. Since surface defects in the form of cracks were discovered in a layer of insulating material that was deposited on a metal layer and furthermore, SEM images showed that the metal layer has exploded, the cause of defect formation may very well be due to electrostatic discharge instead of mechanical stresses in the insulating layer. To verify the causes for surface defects, the electrostatic fields in two separate scrubbers, i.e., a first scrubber which does not present the surface defect problem and a second scrubber which has exhibited a high surface defect rate, were measured in a wet scrubber with the scrubber jet turned on. A higher electrostatic field was measured in the second scrubber chamber. It presents a strong correlation between the electrostatic discharge and the surface defects. The electrostatic field was reduced after an electrically conductive chuck was used in the second scrubber which did not stop the surface defect formation. However, the possibility that surface defects are caused by electrostatic discharges still cannot be ruled out since electrostatic discharges may take place instantaneously when a water jet touches the wafer. In such instance, an electrical charge flows to the conductive chuck so rapidly that the measuring device can not measure a change in the electrostatic discharge. As a result, the true cause for the surface defects, i.e., whether by mechanical stresses imposed in a scrubber clean process or by an electrostatic discharge from the metal layer underneath cannot be ascertained.
It is therefore an object of the present invention to provide a method for determining a cause for defects in a film layer deposited on a wafer surface that does not have the drawbacks or shortcomings of the existing measurement techniques.
It is another object of the present invention to provide a method for determining a cause for defects in a film layer deposited on a wafer surface as due to mechanical stresses or due to electrostatic discharge.
It is a further object of the present invention to provide a method for determining a cause for defects in a film layer deposited on a wafer surface by first eliminating a possible cause of electrostatic discharge.
It is another further object of the present invention to provide a method for determining a cause for defects in a film layer deposited on a wafer surface by substituting a metal layer deposited under an insulating material layer with a second insulating material layer.
It is still another object of the present invention to provide a method for determining a cause for defects in an insulating material layer deposited on a wafer surface by substituting an aluminum layer with a silicon nitride layer underneath the layer of insulating material.
It is yet another object of the present invention to provide a method for determining a cause for defects in a film layer deposited on a wafer by first substituting a metal layer underneath the film layer with a second insulating layer and then scanning a water jet across a top surface of the film layer to detect any defects formed by the water jet pressure.
It is still another further object of the present invention to provide a method for testing a wafer that has a metal conductive layer and an inter-metal dielectric layer sequentially deposited on top by substituting the metal conductive layer with a second insulating material layer and then injecting a water jet of at least 50 kg/cm.sup.2 pressure on top of the inter-metal dielectric layer to detect defects formed by the water jet pressure.
It is yet another further object of the present invention to provide a method for testing a wafer which has an aluminum layer and an oxide layer sequentially deposited on top by first substituting the aluminum layer with a silicon nitride layer and then injecting a water jet of at least 50 kg/cm.sup.2 pressure on top of the oxide layer to detect any formation of stress cracks caused by the high pressure water jet.